Throughout its history, the golf ball has undergone an extensive evolution in an effort to improve its play-related characteristics, e.g., durability, distance, and control. Modern day golf balls can be classified as one-piece, two-piece, and three-piece (also known as "wound") balls. One-piece balls are formed from a homogeneous mass of material with a dimple pattern molded therein. One-piece balls are inexpensive and very durable, but do not provide great distance because of relatively high spin and low velocity.
Two-piece balls are the most popular types of ball in use today. They are made by molding a cover around a solid core. Conventionally, both two-piece and three-piece golf balls are made by molding covers about cores in one of two ways: by injection molding of fluid cover stock material around a core which is held in a retractable pin mold; or by compression molding preformed half-shells about the core. The preformed half-shells are formed by injecting fluid cover stock material into half-shell molds and solidifying the cover stock material into a corresponding shape.
Golf ball cores, whether wound or solid, typically measure from 1.4 to 1.6 inches (3.5 to 4.1 cm) in diameter. The cover is molded about the core to form a golf ball having the minimum United States Golf Association (USGA) specified diameter of 1.68 inches (4.3 cm). Typically, the cover has a thickness of about 0.04 inches (0.1 cm). Two-piece balls typically have a hard "cutproof" cover which gives a longer distance ball, but which has lower spin rates, resulting in a decreased ability to control the ball.
Three-piece or wound balls are made by molding a cover about a wound core. The core is typically made of rubber and can be solid, semi-solid or have a fluid, e.g., liquid-filled center. A wound core is prepared by winding a thin thread of elastic material about the center core. The wound core is then covered with a durable cover material. Wound balls are generally softer and provide more spin, resulting in increased control over the ball, but such balls typically travel a shorter distance than that traveled by a two piece ball. As a result of their more complex construction, wound balls generally require a longer time to manufacture and are more expensive to produce than two-piece balls.
The covers of golf balls sold today are made from a variety of materials, such as balata, SURLYN.RTM. and IOTEK.RTM.. Balata, i.e., a natural or synthetic trans-polyisoprene rubber is the softest of these cover materials. For many years, balata was the standard cover stock material for most golf balls. Balata covered balls are favored among professionals and more advanced amateur players because the softness of the cover allows the player to achieve spin rates sufficient to more precisely control ball direction and distance, particularly on shorter approach shots.
However, balata covered balls are expensive and less durable as compared to the other covering materials. In particular, balata covered balls are subject to nicks or cuts as a result of a mis-swung golf club, which is not uncommon with the average recreational golfer. Such nicks or cuts detract from the flight characteristics of such balls, rendering them of little use. Accordingly, cover compositions have been developed in an attempt to provide spin rates and a feel approaching those of balata covered balls, while also providing a golf ball with a higher durability and overall distance.
In the mid-1960s, E.I. DuPont de Nemours and Co. discovered a species of resins known as ionomer resins which, to a large extent, have replaced balata as a cover stock material. Chemically, these ionomer resins are a copolymer of an olefin and an alpha, beta ethylenically unsaturated carboxylic acid with 10-90% of the carboxylic acid groups neutralized by a metal ion. See U.S. Pat. No. 3,264,272, issued Aug. 2, 1966. Today, commercially available ionomer resins include, for example, copolymers of ethylene and methacrylic or acrylic acid, e.g., sold by E.I. DuPont de Nemours and Co. under the trademark "SURLYN.RTM." and by the Exxon Corporation under trademark "IOTEK.RTM.". These ionomer resins are distinguished by the type of metal ion, the amount of acid, and the degree of neutralization. In addition, Chevron Chemical Co. more recently introduced a family of ionomers produced from ethylene acrylate based copolymers, sold under the trademark "IMAC.RTM.".
Dunlop Rubber Company obtained the first patent on the use of SURLYN.RTM. for the cover of a golf ball, i.e., U.S. Pat. No. 3,454,280 issued Jul. 8, 1969. Since then, there have been a number of disclosures on the use of these ionomer resins in the cover composition of a golf ball. See, for example, U.S. Pat. Nos. 3,819,768 issued Jun. 25, 1974; 4,323,247 issued Apr. 6, 1982; 4,526,375 issued Jul. 2, 1985; 4,884,814 issued Dec. 3, 1989; and 4,911,451 issued Mar. 27, 1990. However, while SURLYN.RTM. covered golf balls as described in the preceding patents possess virtually cutproof covers, they have inferior spin and feel properties as compared to balata covered balls.
In November, 1986, DuPont introduced a sodium and zinc ionomer resin having a low flexural modulus and suggested using and blending the same with other ionomer resins for making a golf ball cover. Golf ball covers made from these low flexural modulus ionomer resins have improved spin and feel characteristics but relatively low velocity.
In Dec., 1986, DuPont introduced a lithium ionomer resin which was a copolymer of ethylene and methacrylic acid. These lithium ionomer resins have a very high flexural modulus, typically about 60,000 psi (415 MPa). DuPont suggested that lithium ionomer resins could be used to produce a golf ball cover which would be more cut resistant and harder than a cover made with either sodium or zinc ionomer resins. DuPont also suggested that a golf ball having a cover made from a lithium ionomer resin would go farther, have a higher coefficient of restitution and be less prone to cutting (i.e., more durable) than a golf ball made from other known ionomer resins such as sodium and zinc ionomer resins and blends thereof. DuPont further suggested that lithium ionomer resins could be used in blends with other ionomer resins where they can impart better cut resistance to those other resins.
The USGA has promulgated a rule that no golf ball shall have an initial velocity that exceeds 255 feet (78 m) per second, i.e., 250 feet (76 m) per second with a 2% tolerance. Golf balls with covers made from ionomer resins with a low flexural modulus are woefully below this maximum and, as should be appreciated, all golf ball manufacturers strive to come as close as possible to this limit.
In various attempts to produce an ideal golf ball, the golfing industry has blended hard ionomer resins (i.e., those ionomer resins having a hardness of about 60 to 66 on the Shore D scale as measured in accordance with ASTM method D-2240) with a number of softer polymeric materials, such as softer polyurethanes. However, the blends of the hard ionomer resins with the softer polymeric materials have generally been unsatisfactory in that these balls exhibit numerous processing problems. In addition, the balls produced by such a combination are usually short on distance.
In addition, various hard-soft ionomer blends, i.e., mixtures of ionomer resins which are significantly different in hardness and/or flexural modulus, have been attempted. U.S. Pat. No. 4,884,814 discloses the blending of various hard methacrylic based ionomer resins with similar or larger quantities of one or more "soft" ionomer methacrylic acid based ionomer resins (i.e., those ionomer resins having a hardness from about 25 to 40 as measured on the Shore D scale) to produce relatively low modulus golf ball cover compositions that are not only softer than the prior art hard ionomer covers but also exhibit a sufficient degree of durability for repetitive play. These relatively low modulus cover compositions were generally comprised of from about 25 to 70 percent of hard ionomer resins and from about 30 to about 75 percent of soft ionomer resins.
U.S. Pat. No. 5,324,783 to Sullivan discloses golf ball cover compositions comprising a blend of a relatively large amount, e.g., 70-90 wt. %, of hard ionomer resins with a relatively low amount, e.g., 10 to about 25-30 wt. %, of soft ionomers. The hard ionomers are sodium or zinc salts of a copolymer of an olefin having from 2 to 8 carbon atoms and an unsaturated monocarboxylic acid having from 3 to 8 carbon atoms. The soft ionomer is a sodium or a zinc salt of a terpolymer of an olefin having from 2 to 8 carbon atoms, methacrylic acid and an unsaturated monomer of the acrylate ester class having from 1 to 21 carbon atoms.
In order to approximate the characteristics of balata covered balls at lower cost, the art has developed balls having a variety of cover compositions. There are more than fifty commercial grades of ionomers available from DuPont and Exxon with a wide range of properties which vary according to the type and amount of metal cations, molecular weight, composition of the base resin (i.e., relative content of ethylene and methacrylic and/or acrylic acid groups) and additive ingredients such as reinforcements, etc. As noted above, these prior art compositions have a considerably higher cut resistance and durability as compared to balata covered balls. A great deal of research continues in order to develop golf ball cover compositions exhibiting not only improved impact resistance and carrying distance properties produced by the "hard" ionomeric resins, but also the playability (i.e. "spin") characteristics previously associated with the "soft" balata covers, properties which are still desired by the more skilled golfer.
However, despite numerous attempts to replicate the performance of balata covered balls, the golf ball cover compositions of the prior art generally suffer from low spin rates which makes them difficult to control near the greens. Further, such balls tend to have relatively poor "click" and feel as compared to the balata covered balls. Additionally, many of the prior art golf ball cover compositions are made with low flexural modulus ionomer resins which have improved spin and feel characteristics, but relatively low velocity, which results in shorter overall distance.
Consequently, a need exists for a golf ball cover composition which provides spin rates and a feel more closely approximating those of balata covered balls, while also providing as high or a higher degree of durability than that provided by the balls presently available or disclosed in the prior art.
This invention teaches a new route to produce polymers with ionomeric character by selectively carrying out hydrolysis or saponification on copolymers to produce compositions useful in golf balls and their covers. The new cover composition can contain binary, ternary or higher blends of metal cations used to neutralize the polymer. The new family of polymers with ionomeric character can be blended with other polymers, such as SURLYN.RTM., IOTEK.RTM. and IMAC.RTM. ionomers to produce golf balls and golf ball covers with desirable properties. The golf ball composition can be used for both solid and wound construction balls.
Hydrolysis or saponification of alkyl acrylate units in a crosslinkable polymer chain is disclosed by Gross in U.S. Pat. No. 3,926,891, issued Dec. 16, 1975. This is accomplished by dissolving the polymer in an aqueous alkali metal hydroxide solution and then heating. The product is recovered by coating the solution onto a substrate and evaporating the water or by extruding the solution into a non-solvent. In U.S. Pat. No. 3,970,626, issued Jul. 20, 1976, Hurst discloses heating a mixture of an alkali metal hydroxide, a thermoplastic ethylene-alkyl acrylate copolymer and water to saponify the acrylate units and form an aqueous emulsion. This emulsion can be used as such, partially dried to a paste or moist solid, or fully dried to solid form.
A different approach to hydrolysis or saponification of an ethylene-alkyl acrylate copolymer is disclosed by Kurkov in U.S. Pat. No. 5,218,057, issued Jun. 8, 1993. There, the copolymer is mixed with an aqueous solution of an inorganic alkali metal base at a temperature sufficient for saponification to take place and at which the copolymer undergoes a phase change. Typically, the copolymer would be molten when mixed with the aqueous solution.
All of these prior methods require that the polymer component be in contact with water, either by conducting the reaction in an aqueous medium or by adding an aqueous solution to the polymer. Processes of this nature pose several disadvantages, however. First, it is difficult to remove water from the hydrolyzed or saponified polymer product. The polymer product is in the form of a salt that has a more polar nature than the reactant acrylate ester, and so is more likely to associate with or hydrogen bond to a polar solvent like water. The energy required to remove a highly interacting polar solvent like water is much greater than for a nonpolar or weakly polar organic solvent. Second, it is important to remove water from the ionomer product because the presence of water can have detrimental effects on ionomer mechanical properties imparted by the polar ionic domains, which act as the effective crosslink sites. Residual water weakens the ionic interactions within these domains, thereby reducing the mechanical property benefits the domains impart. Finally, incomplete removal of water can lead to difficulty in later fabricating steps where the product ionomer is reheated and shaped, e.g., into golf ball covers. Residual water can cause undesirable irregularities and imperfections on the surface of fabricated articles by the formation of blisters. Residual water within fabricated polymer articles can lead to void formation and even foaming with a concomitant undesirable influence on the mechanical properties, load bearing capacity and durability of the fabricated articles.
Melt state neutralization of an ethylene-acrylic acid copolymer by a solid, solution or slurry of an alkali metal salt is disclosed by Walter in U.S. Pat. No. 3,472,825, issued Oct. 14, 1969. In the examples provided, hydrolysis is accomplished by mixing an alkali hydroxide with copolymer at constant temperature either in a Banbury mixer or on a two roll mill. Walter does not disclose the use of extrusion type polymer processing apparatus for this neutralization.
McClain, in U.S. Pat. No. 4,638,034, issued Jan. 20, 1987, discloses a process whereby ethylene-acrylic acid copolymers or their ionomers are prepared from ethylene-alkyl acrylate copolymers by saponifying the latter in the melt with metal hydroxides to form an ionomer and a by-product, i.e., alkanol, then optionally acidifying the ionomer to form the free acid copolymer. This process proceeds in the molten state and in the absence of solvent or water, other than the by-product alkanol. Saponification proceeds under non-static mixing conditions, typically with equipment commonly employed in the art of mixing molten polymer materials such as multiroll mills, a Banbury mixer or a twin screw extruder.
The process disclosed by the '034 reference is, however, incapable of providing optimal product quality since blending and saponifying in a single operation as taught by the subject reference leads to rapid hydrolysis, with a concurrent rapid increase in viscosity. Due to this rapid increase in viscosity, the resultant mixture is non-uniform and therefore the physical properties of products made from this material are not consistent throughout the product.
During the melt state conversion of the alkyl-acrylate copolymer to the metal acrylate copolymer salt, a great decrease in melt flow rate occurs with a corresponding great increase in melt viscosity. While not wishing to be bound by any particular theory, this decreased melt flow rate is thought to occur because of the tendency of the relatively polar ionic salt functionalities formed during the saponification reaction to associate with themselves rather than the relatively nonpolar unreacted alkyl acrylate or comonomer chain segments. Aggregations of salt moieties arising from sidegroups attached to different chains into ionic domains introduces effective crosslink points throughout the molten copolymer. The effective crosslinks, in turn, greatly increase the copolymer melt viscosity and, correspondingly, greatly decrease copolymer melt flow rate.